Generally, a turbo generator includes a compressor, a gas turbine, and a motor generator coupled on the same axis. The compressor and the motor-generator are powered by the gas turbine via combustion gases obtained by burning a fuel with compressed air. More specifically, combustion occurs when the compressed air is supplied to a combustor at the same time when the fuel is sprayed into the combustor, and a turbine is rotated according to the high temperature and high pressure gases generated by the combustion. Then, compressed air and electricity are obtained by the rotation of the compressor and the motor-generator mounted on the same axis of the gas turbine. This kind of turbo generator can be applied to a small generator or to a fuel cell system coupled with a fuel cell.
Conventional turbo generators have several drawbacks in relation with the cooling structure of the motor-generator. The motor-generator requires a cooling structure since the motor-generator is heated by heat generated when: an air current is conducted to the motor-generator through the stator winding unit; the bearings rotate; and from the gas turbine and the compressor adjacent to the motor-generator. In a conventional turbo generator, a desired level of cooling can be achieved since external air can cool a surface of the motor-generator by disposing the air inlet close to the motor-generator and by forming an additional flow channel through which a coolant can flow. These cooling systems, however, not only complicate the overall structure of the device, they also result in low cooling efficiency.